Polymer electrolyte fuel cells are characterized by using a proton conducting polymer membrane as the electrolyte. It is mainly composed of a fuel electrode, an air electrode, and a proton conducting polymer membrane sandwiched between both electrodes. It is an electrochemical device to generate electricity by reacting a fuel and oxygen to generate an electromotive force between the fuel electrode and the oxygen electrode. The proton conducting polymer membrane is sandwiched between the fuel electrode and the air electrode and plays a role as an electrolyte to transport protons into the membrane.
In polymer electrolyte fuel cells, in case that the fuel is hydrogen, hydrogen supplied to the fuel electrode is converted to hydrogen ions H+ (protons) and electrons to give the electrons to the fuel electrode, and the protons pass through the proton conducting polymer membrane to move to the air electrode. The protons that have moved to the air electrode react with oxygen at the air electrode and obtain electrons from the air electrode, thereby producing water. Upon this, the reaction at the fuel electrode is represented by “H2→2H++2e−”, the reaction at the air electrode is represented by “O2+4H++4e−→2H2O”, and the reaction in total is represented by “2H2+O2→2H2O”.
On the other hand, in direct methanol fuel cells, the fuel is methanol, and methanol is reacted with water on the fuel electrode by using a catalyst to convert methanol to carbon dioxide, protons and electrons. Upon this, the reaction at the fuel electrode is represented by “CH3OH+H2O→CO2+6H++6e−”, the reaction at the air electrode is represented by “O2+4H++4e−→2H2O”, and the reaction in total is represented by “2CH3OH+3O2→2CO2+4H2O”. In direct methanol fuel cells, a methanol oxidation electrode catalyst-supported electrode is used as the fuel electrode, and a reduction catalyst-supported electrode is used as the air electrode.
Furthermore, in the present invention, the proton conducting polymer membrane refers to a polymer membrane that transmits protons. Furthermore, the solid electrolyte membrane is a membrane that does not transmit electrons, but transmits only ions and refers to a membrane made of a polymer (resin) having an insulation quality that does not make a short circuit between the negative electrode and the positive electrode.
As the performance requirements of the proton conducting polymer membrane, it is possible to mention that it easily transmits protons, namely, high proton conductivity, that it has a sufficient performance to block transmission (cross leak) of methanol and hydrogen as the fuels and oxygen, that it is superior in strength and heat resistance, and that it is superior in water resistance and chemical stability, etc.
However, proton conducting polymers used up to now as the materials of proton conducting polymer membranes for polymer electrolyte fuel cells have not satisfied all of these performance requirements. Therefore, theoretical potential differences have not been obtained. It has been a major obstacle to the development and the spread of polymer electrolyte fuel cells.
In particular, in direct methanol fuel cells, there becomes problematic a methanol cross over phenomenon in which methanol supplied to the fuel electrode passes through the proton conducting polymer membrane, reaches the air electrode, and reacts with O2 to produce CO2 and H2O. The methanol cross over phenomenon can be prevented by making the proton conducting polymer membrane thicker. If the proton conducting polymer membrane is made thicker, however, there has been a problem that it becomes difficult to allow protons to pass through the proton conducting polymer membrane, that is, the membrane resistance against protons becomes greater, thereby lowering the output of the direct methanol fuel cells.
In the case of polymer electrolyte fuel cells using hydrogen as the fuel, in general, perfluorosulfonic acid series resins are used for proton conducting polymer membranes, in which water is adsorbed to the surroundings of the sulfonic acid group to form a cluster structure, in which water gathers like a cluster of grapes relative to the sulfonic acid group. The proton conductivity appears by the movement of protons by using a cluster, which is an aggregate of this water, as a channel (medium). Therefore, it is necessary to maintain a sufficient amount of water in a perfluorosulfonic acid series resin in order to make the perfluorosulfonic acid series resin demonstrate a high proton conductivity.
In direct methanol fuel cells, however, there has been a problem that methanol, which is high in hydrophilicity, is dissolved in water making the cluster and more easily passes through the membrane by maintaining more water in the perfluorosulfonic acid series resin, resulting in the occurrence of methanol cross over phenomenon.
Thus, in direct methanol fuel cells, in the case of using a perfluorosulfonic acid series resin for the proton conducting polymer membrane, there has been a problem that methanol, which is high in hydrophilicity, is dissolved in water of the cluster of the perfluorosulfonic acid series resin and passes through the proton membrane, thereby lowering the output of direct methanol fuel cells.
A technique for suppressing such methanol cross leak, namely, methanol cross over phenomenon is disclosed in Patent Publication 1 or Patent Publication 2.
Patent Publication 1 discloses an ordinary temperature type, acid, direct methanol fuel cell characterized by that, as solid electrolyte membranes between the positive electrode and the negative electrode, polystyrene graft polymerization membranes having a sulfonic acid group in the chemical structure are used on both sides of a cation-exchange membrane and that at least two of the cation-exchange membranes are disposed by lamination.
Furthermore, Patent Publication 2 discloses a fullerene derivative-containing proton conducting membrane that is superior in performance to block transmission of methanol and in proton conductivity, an electrolyte membrane, a membrane-electrode assembly, and an electrochemical device.
In the direct methanol fuel cell described in Patent Publication 1, the membrane thickness is made thick by laminating cation-exchange membranes, thereby blocking the transmission of methanol. By laminating cation-exchange membranes, the transmission of methanol in the cation-exchange membranes is lowered. However, the lamination of cation-exchange membranes increases the membrane's resistance against protons. Therefore, there has been a concern that the output of the direct methanol fuel cell is lowered, thereby lowering the power generation capacity. In addition, there has been a concern that the production process becomes complicated by laminating a plurality of cation-exchange membranes, thereby making the membrane-electrode assembly have a high price.
Thus, in polymer electrolyte fuel cells, there has been a problem that, when the membrane thickness of the proton conducting polymer membrane is made thick in order to prevent the cross leak in which methanol or hydrogen supplied to the fuel electrode passes through the proton conducting polymer membrane and moves to the air electrode side, it becomes difficult to transmit protons, that is, the membrane resistance against protons becomes large, thereby lowering the output of polymer electrolyte fuel cells.
The fullerene derivative-containing complex membrane described in Patent Publication 2 is not perfect in terms of prevention of leak of the proton conducting group. Furthermore, there has been a problem that an industrial mass production is difficult due to the use of fullerene as a raw material, and a membrane-electrode assembly to be produced has a high price.
A solid electrolyte membrane as a resin membrane containing a sulfonic acid group as a strong acid group, which is used in polymer electrolyte fuel cells using hydrogen as a fuel, has a high conductance of methanol. This is because diffusion of methanol is accelerated due to a strong maintenance of water in the membrane by a strong hydrophilicity of the sulfonic acid group. Furthermore, a perfluoro resin series electrolyte membrane is formed with micropores capable of transmitting methanol by the cluster structure. Therefore, it has been difficult to say that it is preferable to use that for direct methanol fuel cells.
The solid electrolyte membrane with a suppressed methanol conductance, which is described in Patent Publication 1 or Patent Publication 2, has a hard membrane quality. Thus, it has been difficult to say that it is adhered to the electrodes with good adhesion.
A proton conducting polymer membrane is required to easily transmit protons, namely, a high proton conductance, to be sufficient in capacity for blocking the transmission (cross leak) of methanol and hydrogen as the fuels and oxygen, and to be superior in strength, heat resistance, water resistance and chemical stability. When having made a membrane-electrode assembly, it is required that adhesion between the proton conducting polymer membrane and the electrode plate (in the following, simply referred to as electrode) is satisfactory.